Did you know that Jared Lee Loughner, the suspect in the Arizona shooting spree that left six dead and 14 wounded, including the US congresswoman Gabrielle Giffords, had speculated beforehand on YouTube whether he would be labelled a "terrorist"? He needn't have worried. Loughner has yet to be described in such terms by the authorities or the media. "Loner"? Yes. "Extremist"? Yes. Terrorist? No.

Perhaps, you might say, it's because we have difficulty agreeing on a definition of terrorism, despite the Terrorist Tourette syndrome that so many of our politicians and commentators have suffered from in recent years. "Most of the time, if something looks like a terrorist and makes a noise like a terrorist, it's a terrorist," remarked the then British ambassador to the UN, Sir Jeremy Greenstock, in the wake of the September 11 attacks. If only it were that simple. Terrorism has long been a contested and loaded term. A 2003 study for the US army quoted a source that counted 109 definitions of terrorism that covered 22 different definitional elements.

Nonetheless, most would agree that the use of violence against civilians for political purposes is a form of terrorism. And section 802 of the USA Patriot Act, passed by a Republican Congress the month after the attacks on the World Trade Centre, explicitly expanded the term "terrorism" to include domestic actions that are "dangerous to human life" and are intended to "intimidate or coerce a civilian population", "influence the policy of a government by intimidation or coercion", or "affect the conduct of a government by mass destruction, assassination, or kidnapping".

Isn't the firing of a bullet into the head of an elected politician in a public place covered by any, or all, of those three criteria? Or does the suspect have to have an Arabic name to be classed as a terrorist these days? Imagine, for a moment, that the shooter outside the Tucson Safeway last Saturday had been a Muslim. Does anyone doubt that accusations of homegrown terrorism, links to al-Qaida and vast Islamist conspiracies wouldn't have come thick and fast?

Compare and contrast the response to the shooting of Giffords to the stabbing of the Labour MP Stephen Timms in his constituency surgery last May. The investigation into his Muslim attacker, Roshonara Choudhry, was conducted by Scotland Yard's counter-terrorism command. The Guardian described the stabbing as "the first terrorist attack to injure someone on the UK mainland since 7 July 2005".

There is a pattern here. The so-called war on terror began after Arab hijackers crashed airliners into the twin towers, killing thousands of civilians. On 18 February, a 53-year-old software engineer named Joseph Stack flew a light aircraft into an IRS building in Austin, Texas, killing one other person and injuring 13. Despite posting an anti-government suicide note on the internet claiming, "Violence not only is the answer, it is the only answer", the White House press spokesman did not describe the attack as an act of terrorism – and neither did the Austin police chief or the department of homeland security. He was a "loner", a "lone wolf", a "pilot" – or, in the words of his daughter and several conservative radio hosts, a "hero".

On 9/11, hijackers also attacked the Pentagon, killing 125 people inside the building. They were rightly described as "terrorists". On 4 March last year, a computer programmer critical of the government, named John Patrick Bedell, attacked the Pentagon, shooting two police officers. News reports described him only as a "gunman" and the "Pentagon shooter". In 2003, the convictions of six Islamists living in Lackawanna, New York, prompted frenzied talk of "terrorist sleeper cells" inside the US; so too did the conviction of six Islamists for conspiring to stage an attack on US troops stationed at Fort Dix, New Jersey, in 2008. Last April, nine members of the Michigan-based "Hutaree" (or "Christian Warriors") group, arrested and charged with conspiring to kill local police officers using "weapons of mass destruction", were described simply as members of a "militia". The list goes on.

So why the double standards? I suspect it is because of the lazy and pernicious notion, peddled and popularised by neoconservatives and Islamophobes, that not all Muslims may be terrorists, but certainly all terrorists are Muslims. In fact, nothing could be further from the truth. The authorities' own data contradicts their public rhetoric. FBI figures show between 2002 and 2005 there were 24 acts of terrorism recorded in the US; 23 of those incidents were carried out by non-Muslim,"domestic terrorists".

The picture is not dissimilar on this side of the Atlantic. The EU's little-noticed Terrorism Situation and Trend Report revealed that in 2009 there were "294 failed, foiled, or successfully executed attacks" in six European countries. The vast majority of these attacks – 237 – were carried out by separatist groups, such as the Basque Eta.

A further 40 terrorist incidents were by leftwing and anarchist groups, while rightwing terrorists were held responsible for four of the attacks. How many attacks were classed as having been committed by Islamists? Just one. That is the same number of attacks as committed by the CAV (Comité d'action viticole), a French group that campaigns against the import of foreign wine. In 2008, again according to EU figures, there were no attacks by Islamists at all. In 2007, there were four Islamist attacks out of 583.

Is it fair, then, to assume or pretend that terrorism is the sole or exclusive preserve of Muslims? Let's be clear: it would be foolish and dangerous to dismiss or downplay the threat from Islamist terrorism, to Europe or the US. There is a real and present danger from al-Qaida, its affiliates and self-radicalised fellow travellers such as Choudhry. And Islamist terrorists, unlike their separatist, far-right or anarchist counterparts, are bent on high-profile, mass-casualty "spectaculars".

But terrorism is a tactic and, as such, is agnostic: those who murder civilians to make a political point are called Joseph as well as Muhammad. Supporters of the "war on terror" tend to argue that such definitional double standards are unavoidable – yet the result of inconsistency on the part of politicians, journalists and law-enforcement officials has been to empty the term virtually of all meaning, and render it useful only for crude propaganda purposes. The lesson of the Arizona shooting should be clear: either we use the term "terrorist" with consistency, or we don't use it at all. There is no room for double standards.